In general, laminar flowmeters which use the linear relationship between a fluid flow rate and characteristics of the fluid, such as the change in pressure or rate of heat transfer, are known. The details of their operation are well known to those of ordinary skill in the art. Examples of flow meters are described in U.S. Pat. Nos. 4,118,973 issued to Tucker et al.; No. 4,800,745 issued to Korpi; and No. 4,427,030 issued to Jouwsma. As is also known, to produce and maintain laminar flow through such a measuring device, certain criteria have to be met. Extensive studies have been conducted to characterize these properties and the most widely used characterization is referred to as the Reynolds number. It has been determined that for smooth pipes the transition from a laminar to turbulent boundary layer occurs when the ratio of .rho.D.nu./.eta. becomes larger than approximately 2,000, where .nu. is the average velocity of the fluid, D is the characteristic linear dimension of the pipe, .rho. is the density of the fluid, and .eta. is the viscosity of the fluid. Preferably, to ensure laminar flow, the Reynolds number should be less than 1000.
Due to difficulties in manufacturing round pipes as laminar flow elements, attempts have been made to make laminar flow elements designed around rectangular channels. When dealing with rectangular channels, the parameters used to calculate the Reynolds number remain the same, except that D is defined as the equivalent diameter by convention. Equivalent diameter, in turn, is defined as four times the hydraulic radius which is the cross-sectional area divided by the wetted perimeter. Also, the pressure drop across rectangular laminar flow elements can be calculated as 32 .eta..nu.L/D.sup.2, where .eta. is the viscosity of the fluid, .nu. is the average velocity; L is the length of the channel and D is the equivalent diameter.
A difficulty in maintaining laminar flow arises when one tries to create substantial pressure drop in the flow channel. Substantial pressure drop is desired so that conventional electronics can be used to measure that pressure drop with acceptable accuracy. One way to maintain laminar flow and create substantial pressure drops at given flow rates is to make the depth of the rectangular channel substantially smaller than its length.
To enhance the usefulness of a flowmeter, it should be capable of accurately measuring the flow rate for different quantities of flow. For example, an air flowmeter that can accurately measure flow rate of 200 cc per minute, and can easily be modified to measure 1 liter per minute with the same accuracy is more in demand than a single-purpose flowmeter that is only designed for 200 cc/min. This leads to the definition of the full-scale flow rate for each flowmeter, that is, the flow rate at which the flowmeter can provide its best performance. This means that a flowmeter designed for measurement of 200 cc/min full-scale is not appropriate for use at 1 LPM, for the reason that at higher flow rate laminar flow is not ensured due to the increased mean velocity and concomitantly increased Reynolds number. A 200 cc/min flowmeter is also not appropriate for accurate measurement at substantially lower flow rates, e.g., 50 cc/min, as this requires more amplification of the signal, and hence inevitable amplification of noise.
For the reasons presented here, it is desirable to operate the flowmeter at its specified full scale flow rate. It is therefore advantageous to provide a mechanism that allows for the change in the full-scale flow rate of a flowmeter and hence optimizing the performance for different rates of flow.
To adjust the full scale flow rate, Tucker proposes a mechanism in which a number of plates with rectangular grooves are stacked on top of each other. Jouwsma proposes a method in which grooves are formed in metal disks and the metal disks are placed onto each other. Korpi proposes a conical shaped laminar flow element which is molded out of plastic and has multiple parallel channels. These channels are blocked by webbing which can be removed to change the cross section of the primary passage. While these methods provide for adjustment in the full scale range of flow, each has drawbacks and disadvantages.